1. Field of the Invention
This invention relates to hot dip metallic coating of steel strip and sheet stock and more particularly to a method of preliminary treatment of the surfaces of the stock in a sulfur-containing atmosphere to develop initially a sulfur and oxygen rich film, to preserve this film during further heating, and to reduce the film while cooling the stock prior to immersion thereof in a molten coating metal bath. The invention has utility in the coating of carbon steels, low carbon rimmed steels, low carbon aluminum killed steels, and low alloy steels by molten coating metals such as zinc, zinc alloys, aluminum, aluminum alloys, and terne. Low alloy steels which may be treated by the process of the invention contain up to about 3% aluminum, up to about 1% titanium, up to about 2% silicon, or up to about 5% chromium, and mixtures thereof, with the remainder of the composition typical of carbon steel, as defined by Steel Products Manual, Carbon Sheet Steel, page 7 (May 1970) published by American Iron and Steel Institute. Aluminum killed steels include typical low carbon steel as defined above containing from about 0.03% to about 0.06% acid-soluble aluminum.
2. Description of the Prior Art
In the fluxless hot dip metallic coating of steel strip and sheet stock it is necessary to subject the surfaces to a preliminary treatment which provides a clean surface free of iron oxide scale and other surface contaminants, and which is readily wettable by the molten coating metal in order to obtain good adherence. Two types of in-line anneal preliminary treatments are in common use in this country, one being the so-called Sendzimir process or oxidation-reduction practice (disclosed in U.S. Pat. Nos. 2,110,893 and 2,197,622), and the other being the so-called Selas process or high intensity direct fired furnace line (disclosed in U.S. Pat. No. 3,320,085 to C. A. Turner, Jr.).
In the Sendzimir process steel strip or sheet stock is heated in an oxidizing furnace (which may be a direct fired furnace) to a temperature of about 370.degree.-485.degree. C. without atmosphere control, withdrawn into air to form a controlled surface oxide layer varying in appearance from light yellow to purple or even blue, introduced into a reduction furnace containing a hydrogen and nitrogen atmosphere wherein the stock is heated to about 735.degree.-925.degree. C. and the controlled oxide layer is completely reduced. The stock is then passed into a cooling section containing a protective reducing atmosphere, such as hydrogen and nitrogen, brought approximately to the temperature of the molten coating metal bath, and then led beneath the bath surface while still surrounded by the protective atmosphere.
In the Selas process steel strip or sheet stock is passed through a direct fired preheat furnace section, heated to a temperature about 1315.degree. C. by direct combustion of fuel and air therein to produce gaseous products of combustion containing at least about 3% combustibles in the form of carbon monoxide and hydrogen, the stock reaching a temperature of about 425.degree.-705.degree. C. while maintaining bright steel surfaces completely free from oxidation. The stock is then passed into a reducing section which is in sealed relation to the preheat section and which contains a hydrogen and nitrogen atmosphere, wherein it may be further heated by radiant tubes to about 425.degree.-925.degree. C. and/or cooled approximately to the molten coating metal bath temperature. The stock is then led beneath the bath surface while surrounded by the protective atmosphere. The process may optionally include holding the stock at a selected temperature in a reducing atmosphere after reaching maximum temperature in the radiant tube section.
U.S. Pat. No. 3,936,543 issued Feb. 3, 1976, to F. Byrd et al., discloses an improvement in the Selas process, resulting in higher combustion efficiency and better production rates, wherein strip and sheet stock is heated to about 540.degree.-705.degree. C. in a direct fired preheat furnace section heated to at least about 1205.degree. C. and containing gaseous products of combustion ranging from about 3% by volume oxygen to about 2% by volume excess combustibles in the form of carbon monoxide and hydrogen, followed by heating in a reducing section containing at least about 5% hydrogen by volume to a temperature of at least about 675.degree. C. Preferably the preheat furnace atmosphere contains 0% oxygen and 0% excess combustibles, i.e., perfect combustion.
In all prior art processes for preliminary treatment of steel strip and sheet surfaces which are exposed to atmospheres of direct fired furnaces, it has been considered that the presence of even small amounts of sulfur in the atmosphere would be highly deleterious. Accordingly, substantially sulfur-free fuel such as natural gas has been prescribed for use in such furnaces. However, natural gas shortages have made it necessary to consider alternative sources of fuel. In a steel mill having coke ovens, the use of coke oven gas as a fuel source would be an obvious choice except for the fact that raw coke oven gas ordinarily contains about 300 to 500 grains of sulfur per 100 cubic feet of gas, the sulfur being present primarily as hydrogen sulfide with a small amount of organic sulfur compounds. Although the gas can be easily scrubbed to a sulfur level of about 75 to 100 grains per 100 cubic feet, and more recently even to a level of about 25 to 40 grains per 100 cubic feet, it has nevertheless been generally considered that preliminary treatment methods involving exposure of steel strip surfaces to atmospheres containing products of combustion could not tolerate even the lower sulfur levels of scrubbed coke oven gas. Accordingly, it has been feared that curtailment of natural gas supply would force the shut-down of coating lines equippd with direct fired furnaces for preliminary treatment of steel strip and sheet material.